Conventional disposable packaging and service items such as containers, trays, plates, bowls, and others, are commonly made from polystyrene or other synthetic hydrophobic plastics and also from paper or paperboard coated commonly with polyethylene. These materials are durable and are moisture-resistant and grease-resistant. The items are produced by industry in great quantities and are relatively inexpensive. After usage, these biostable packaging items are discarded in the environment and therefore create serious ecological problems. The expected lifetime of polystyrene packaging, for example, is approximately 500 years. Further, polystyrene and other plastics are made from nonrenewable petroleum resources. Recycling of plastic items is an expensive process. Moreover, recycled plastics are prohibited as a material for production packaging for food. In China, for example, the use of polystyrene foam in disposable packaging is illegal.
The pressure to use biodegradable disposable packaging materials has steadily increased in the last decade. The main approach in the art has been the manufacture of compostable and inexpensive food-service items such as containers, trays, plates and bowls based on biodegradable and natural raw materials such as starch, cellulose, proteins, etc. U.S. Pat. No. 4,863,655 issued in 1989 to N. Lacourse et al., U.S. Pat. No. 5,362,776 issued in 1994 to S. Barenberg, et al., U.S. Pat. No. 6,030,673 in 2000 to P. J. Andersen, et al. and U.S. Pat. No. 6,723,264 issued in 2004 to H. Bussey, Jr. disclose materials containing a starch binder, reinforcing cellulose fibers, and additives for obtaining compostable and biodegradable food packaging. Production methods of the disclosed materials are also described.
However, packaging based on natural biodegradable materials has several disadvantages, the most important being susceptibility to water, grease, and various other liquids. A starch binder is water-soluble and penetrable to grease. Cellulose and proteins are hydrophilic and therefore swell in water. When exposed to water or significant amounts of water vapor, these packaging items lose form-stability and become susceptible to breakage. Water, grease, and other liquids, as well as gases and vapors, easily penetrate natural materials because of high porosity of these materials. It is known that paper and other natural materials contain pores with diameters of dozens of microns (Dodson C. T, et al, Journal of Pulp and Paper Science, 1996, 22, J165-169; Sampson W. W., Journal of Materials Science, 36, 5131-5135).
Currently, various approaches are proposed to solve the problem of moisture and grease susceptibility of packaging made from natural materials. One approach is to protect the packaging from contact with water or other liquids by applying hydrophobic films onto surfaces of the packaging material and forming a laminated or coated structure in which a liquid-sensitive core is sandwiched between layers of synthetic hydrophobic plastics or resins.
Such protected layers may contain various compounds, e.g., polyolefins and additives (U.S. Pat. No. 5,296,307 issued in 1994 to B. Bernstein), copolymers of olefins and unsaturated carboxylic acids, and pigments (U.S. Pat. No. 3,970,629 issued in 1976 to N. Izaki, et al.), and a mixture of polyvinyl chloride and ethylene-acrylic copolymer (U.S. Pat. No. 4,365,029 issued in 1983 to R. Reizer). U.S. Pat. No. 5,053,268 issued in 1991 to R. Ehara, et al. discloses paper containing sheets of synthetic film and paper bonded with a urethane adhesive. All these materials are water- and grease resistant but are not biodegradable.
U.S. Pat. No. 3,985,937 issued in 1976 to R. Fife, U.S. Pat. No. 4,117,199 issued in 1978 to H. Gotoh, et al., U.S. Pat. No. 4,395,499 issued in 1983 to J. Rosenski, et al., U.S. Pat. No. 4,657,821 issued in 1987 to S. Ura, et al., U.S. Pat. No. 5,587,246 issued in 1996 to H. Tsutsumi, et al., and U.S. Pat. No. 6,255,375 issued in 2001 to J. Michelman relate to protection of cellulose substrates and disclose hydrophobic coatings that contain synthetic rubbers, polyvinyl esters, polyacrylates, various copolymers, paraffin wax, organic acids, fillers, and other additives. The coated materials are waterproof; however their biodegradability is low and, therefore, they pollute the environment.
Latexes (aqueous polymer dispersions) cannot be used in protective coatings of hydrophilic natural materials containing cellulose, protein, or starch because of swelling, warping, and dissolving of these natural materials, when they come into contact with an aqueous type of latex during the coating process.
Lamination of cellulose, paper, or other natural packaging materials with synthetic polymers or resins prevents recycling and composting thereof. When a laminate is discarded, it pollutes the environment. Moreover, the valuable raw materials of the laminate are lost.
Some hydrophobic biodegradable polymers and co-polymers such as poly-beta-hydroxyalkanoates (PHA) have been used as laminating layers for cellulose materials (Ioelovich M., Scientific Israel—Technological Advantages, 2001, Vol. 3, No. 2, pp. 152 to 157). These polymers or co-polymers contain ester links that can be destroyed by environmental microorganisms. PHA in a melted state was applied to the cellulose substrate at temperatures above 150 to 160° C. After cooling, a solid water-resistant PHA layer was formed on the cellulose substrate. Nowadays, such polymers and co-polymers of a PHA type are produced in small amounts from rare raw materials. The manufacturing technology is complicated, and thus the cost of such materials is high (tens of USD per kg). It should be pointed out that the biodegradation process of PHA is relatively slow. For example, destruction of PHA in soil takes more than eight months. Furthermore, the application of PHA to cellulose substrate at the above-mentioned temperatures is associated with thermal destruction of the substrate. The use of PHA or other biodegradable polymers as protective coatings is very expensive, and the application, itself, is difficult. In other words, use of these polymers increases manufacturing cost of the final packaging product. Films of biodegradable polymers usually have poor protective and mechanical properties. Moreover, natural packaging material laminated or coated with biodegradable polymer films cannot be recycled.
The coating of cellulose substrate by aqueous dispersions (types of latex) of PHA is known in the art (U.S. Pat. No. 5,350,627 issued in 1994 to S. Nemphos, et al.). The cellulose substrate is impregnated with PHA latex and is then dried at room temperature and pressed for a few minutes at 100 to 140° C., i.e., at a temperature below the melting point of PHA. Because of low temperature and short period of time, thermal destruction of cellulose can be avoided. However, since PHA particles do not melt, it is not possible to obtain a monolithic and homogeneous coating. As a result, the coated paper has loose, porous polymer layers and therefore has low resistance to penetration of water and other liquids. Another disadvantage of the above-mentioned method is the necessity to treat the paper with a dilute aqueous dispersion of PHA (5 to 20% or 50 to 200 g/l). This causes considerable swelling, warping, and possible mechanical damage of the initial hydrophilic and porous cellulose substrate during later steps in the production process. Moreover, PHA-dispersion is an expensive product.
Also, it should be noted that use of nano-cellulose particles for the preparation various nano-compositions is known. Current methods for the production of nano-cellulose and nano-cellulose-based compositions consist of hydrolysis of amorphous domains of elementary fibrils with high concentrated (60 to 65%) sulfuric acid and subsequent mechanical or ultrasound disintegration (U.S. Pat. No. 6,117,545 issued in 2001 to J. Cavaille, et al, U.S. Pat. No. 6,103,790 in 2000 to J. Cavaille; Ono, et al, U.S. Pat. No. 6,541,627 issued in 2003 to H. Ono, et al.; and Hubbe, et al, BioResources, 2008, 3 (3), pp. 929 to 980). The main disadvantages of these methods for producing cellulose nano-particles are the use of acids of high concentration, a low yield of nano-particles, and a low solid content of nano-dispersions. Nano-composites produced by mixing a diluted water dispersion of nano-cellulose with polymer latex have poor properties due to provision of a weak van der Waals interaction only between the cellulose nano-particles and the polymer binders.
Thus, despite availability of various manufacturing methods for protective coatings, there remains a strong belief in the need for a new and improved production process of biodegradable nano-compositions for protective coatings of natural packaging materials.